night vision

vision

[vizh´un]

the faculty of seeing; called also sight. adj., adj vis´ual. The basic components of vision are the eye itself, the visual center in the brain, and the optic nerve, which connects the two. (See also Plate 17.)

How the Eye Works. The eye works like a camera. Light rays enter it through the adjustable iris and are focused by the lens onto the retina, a thin light-sensitive layer which corresponds to the film of the camera. The retina converts the light rays into nerve impulses, which are relayed to the visual center. There the brain interprets them as images.

Like a camera lens, the lens of the eye reverses images as it focuses them. The images on the retina are upside down and they are “flipped over” in the visual center. In a psychology experiment, a number of volunteers wore glasses that inverted everything. After 8 days, their visual centers adjusted to this new situation, and when they took off the glasses, the world looked upside down until their brain centers readjusted.

The retina is made up of millions of tiny nerve cells that contain specialized chemicals that are sensitive to light. There are two varieties of these nerve cells, rods and cones. Between them they cover the full range of the eye's adaptation to light. The cones are sensitive in bright light, and the rods in dim light. At twilight, as the light fades, the cones stop operating and the rods go into action. The momentary blindness experienced on going from bright to dim light, or from dim to bright, is the pause needed for the other set of nerve cells to take over.

The rods are spread toward the edges of the retina, so that vision in dim light is general but not very sharp or clear. The cones are clustered thickly in the center of the retina, in the fovea centralis. When the eyes are turned and focused on the object to be seen the image is brought to the central area of the retina. In very dim light, on the other hand, an object is seen more clearly if it is not looked at directly, because then its image falls on an area where the rods are thicker.

Color Vision. Color vision is a function of the cones. The most widely accepted theory is that there are three types of cones, each containing chemicals that respond to one of the three primary colors (red, green, and violet). White light stimulates all three sets of cones; any other color stimulates only one or two sets. The brain can then interpret the impulses from these cones as various colors. Man's color vision is amazingly delicate; a trained expert can distinguish among as many as 300,000 different hues.

Color vision deficiency (popularly called “color blindness”) is the result of a disorder of one or more sets of cones. The great majority of people with some degree of deficiency lack either red or green cones, and cannot distinguish between those two colors. Complete color vision deficiency (monochromatic vision), in which none of the sets of color cones works, is very rare. Most deficiencies of color vision are inherited, usually by male children through their mothers from a grandfather with the condition.

Stereoscopic Vision. Stereoscopic vision, or vision in depth, is caused by the way the eyes are placed. Each eye has a slightly different field of vision. The two images are superimposed on one another, but because of the distance between the eyes, the image from each eye goes slightly around its side of the object. From the differences between the images and from other indicators such as the position of the eye muscles when the eyes are focused on the object, the brain can determine the distance of the object.

Stereoscopic vision works best on nearby objects. As the distance increases, the difference between the left-eyed and the right-eyed views becomes less, and the brain must depend on other factors to determine distance. Among these are the relative size of the object, its color and clearness, and the receding lines of perspective. These factors may fool the eye; for example, in clear mountain air distant objects may seem to be very close. This is because their sharpness and color are not dulled by the atmosphere as much as they would be in more familiar settings.

Impaired Vision. This may consist of loss of visual acuity, visual field, ability to distinguish colors, motion of the eye, or any other function related to sight. (See also blindness.) Farsightedness, or hyperopia, results when the eyeball is shorter than normal and the image focuses behind the retina. Nearsightedness, or myopia, results when the eyeball is longer than usual from front to back, so that the image focuses in front of the retina. astigmatism is impaired vision caused by irregularities in the curvature of the cornea or lens.

Patient Care. Visually handicapped persons who are visiting a clinic for the first time or being admitted to a hospital room require orientation to their environment. Ambulatory patients can be walked around to familiarize them with the location of the bathroom and any other facility they may need to use.

Patients who are in bed following surgery or for therapeutic rest should have articles on their bedside table arranged in the same way all of the time so that they can be found easily. If only one eye is affected, articles should be placed within reach on the unaffected side and persons communicating with the patient also should stand on that side. If peripheral vision is limited, objects and persons should be positioned in the patient's line of vision.

Some patients, especially the elderly, may experience increased sensitivity to glare. Wearing sunglasses outdoors, adjusting the window blinds to deflect the sun, and using indirect lighting can help avoid discomfort. This does not mean that the patient should be in a darkened room. For most, increased illumination makes it easier to see. It is the glare that impairs their vision.

Whenever it is necessary to do something for the visually impaired person, explain beforehand what will be done. This helps reduce confusion and establishes trust in the caregiver. (For patient care, see also blindness.)

Patients with impaired vision may also benefit from such low-vision aids as convex or magnifying lenses that are hand held or mounted on a stand or clipped to the eyeglasses. Adjustable lamps, large-print reading matter, reading stands, writing guides and lined paper, and felt-tipped pens can facilitate reading and writing and improve the quality of life of a person with limited vision.

Top, Anatomy of the eye. Vision is the reception of images by the eye as a result of the passage of light into the eye. Light is focused by the lens on the retina, where it is converted into nerve impulses that are transmitted to the centers in the brain where images are interpreted.

anomalous trichromatic visioncolor vision deficiency in which a person has all three cone pigments but one is deficient or anomalous; it may be either inherited as an X-linked recessive trait or acquired as a result of a retinal, cerebral, systemic, or toxic disorder.

binocular vision the use of both eyes together, without diplopia.

central vision that produced by stimulation of receptors in the fovea centralis.

low vision impairment of vision such that there is significant visual handicap but also significant usable residual vision; such impairment may involve visual acuity, visual fields, or ocular motility.

monochromatic visioncolor vision deficiency in which the person cannot distinguish hues, so that all the colors of the spectrum appear as shades of gray. Popularly known as complete or total color blindness.

1. that in which the visual field is severely constricted. When it is due to organic causes, such as retinitis pigmentosa or glaucoma, the visual field expands as it is tested at increasing distance from a constant object but when it is due to psychogenic disorders, such as conversion disorder or malingering, the field remains constant or contracts as the distance increases.

2. in psychiatry, restriction of psychological or emotional perception to a limited range.

sco·top·ic vis·ion

night vision

n.

Vision in dim light.

night vision

Etymology: AS, niht, night; L, visio, seeing

a capacity to see dimly lit objects. It stems from a chemophysical phenomenon associated with the retinal rods. The rods contain the highly light-sensitive chemical rhodopsin, or visual purple, which is essential for the conduction of optic impulses in subdued light. Night vision is sharpest at the periphery of the retina because of the concentration of rods. Night vision may be diminished by a deficiency of vitamin A, an important component of rhodopsin.

sco·top·ic vi·sion

vision

the faculty of seeing; sight.

The basic components of vision are the eye itself, the visual center in the brain, and the optic nerve, which connects the two. Abnormalities of vision in animals can only be inferred by an assessment of the animal's response to a variety of visual stimuli. The commonly used tests of vision are the menace reflex test, the watching of a moving object and the obstacle test. These can all be performed in subdued light as a test for night blindness.

achromatic vision

vision characterized by lack of color vision.

aphakic vision

vision after lens removal.

binocular vision

the use of both eyes together, without diplopia.

central vision

that produced by stimulation of receptors in the fovea centralis.

day vision

visual perception in the daylight or under conditions of bright illumination.

double vision

diplopia.

half vision

hemianopia.

monocular vision

vision with one eye.

night vision

visual perception in the darkness of night or under conditions of reduced illumination.

panoramic vision

360° vision conferred on grazing herbivora by the lateral placement of their eyes.

peripheral vision

that produced by stimulation of receptors in the retina outside the macula lutea.

Specific forward-looking statements contained in this press release include, but are not limited to, our customer's projected use for our night-vision sensor modules and the Company's entry into high-volume military night-vision markets.

MII is working to solve the cost problem that makes the wide applicability of night-vision and thermal-imaging products impractical, if not impossible," says Battelle Ventures General Partner Kef Kasdin, who has joined the Board of Directors of the company, headquartered in Parsippany, N.

The BA-5367/U is a one ampere-hour, single-cell 3-volt lithium-manganese dioxide battery used in the AN/AVS-6 aviator's night-vision goggles, AN/PVS-5 night vision goggles, AN/PVS-4 night-vision weapons sight, and various other imaging and aiming systems.

All content on this website, including dictionary, thesaurus, literature, geography, and other reference data is for informational purposes only. This information should not be considered complete, up to date, and is not intended to be used in place of a visit, consultation, or advice of a legal, medical, or any other professional.